insert type assertions when narrowing.

R=r
OCL=24349
CL=24913
diff --git a/test/fixedbugs/bug089.go b/test/fixedbugs/bug089.go
index 296e9e6..e3d8ff6 100644
--- a/test/fixedbugs/bug089.go
+++ b/test/fixedbugs/bug089.go
@@ -17,5 +17,5 @@
 func
 main()
 {
-	i2 = e();	// bug089.go:16: fatal error: agen_inter i2i
+	i2 = e().(I2);	// bug089.go:16: fatal error: agen_inter i2i
 }